Abstract: Melatonin (N-acetyl-5-methoxytryptamine, MT), a small-molecule indoleamine widely present in animals and plants, plays an important role in regulating plant growth, development, and stress resistance. Using CiteSpace software, this study conducted a visual analysis of domestic and international literature from 2015 to 2025, revealing a surge of research on exogenous melatonin in the regulation of crop quality. The research progress in this field is systematically reviewed from the perspectives of biosynthesis, quality regulation mechanisms, regulatory mechanisms, and stress resistance improvement. As an exogenous signal, it can stimulate endogenous MT synthesis, with its content dynamically varying in response to environment and plant developmental stages. Its role in quality regulation is particularly evident during fruit maturation. Specifically, exogenous melatonin improves external qualities, such as maintaining fruit color and firmness, and enhances internal qualities by increasing nutrients (e.g., soluble solids and vitamin C) and volatile aromatic compounds. This quality preservation is primarily attributed to the fact that during postharvest ripening and senescence, melatonin maintains crop quality by inhibiting ethylene synthesis, activating the antioxidant system, and maintaining energy metabolism. Furthermore, melatonin significantly contributes to quality preservation under stress. It enhances cold tolerance by inducing proline accumulation and regulating membrane lipid metabolism, mitigates heavy metal and sodium ion toxicity by altering osmotic balance and maintaining ion homeostasis, and improves disease resistance by activating jasmonic acid and salicylic acid signaling pathways and promoting the expression of pathogenesis-related proteins. Based on the current research status, future studies should focus on the following three aspects: 1) elucidating the broad-spectrum mechanism of MT in synergistically improving quality and yield, as well as in stress resistance-enhanced quality; 2) expanding field application research of MT; and 3) strengthening the combined effects of MT with other regulatory substances. This will provide new theoretical foundations and practical guidance for the application of plant melatonin in regulating crop quality and its use in agriculture.